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'[EE] Schottky diode in ICSP circuit'
2005\06\17@174107 by Peter Onion

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I'm looking at TB016 "How to Implement ICSP™ Using PIC16F8X FLASH MCUs"
http://ww1.microchip.com/downloads/en/AppNotes/91016b.pdf

"The MCLR/VPP pin is normally connected to an RC circuit.
The pull-up resistor is tied to VDD and a capacitor
is tied to ground. This circuit can affect the operation of
ICSP depending on the size of the capacitor. It is,
therefore, recommended that the circuit in Figure 1 be
used when an RC is connected to MCLR/VPP. The
diode should be a Schottky-type device."
With that diode in place (anode to capacitor-and-resistor to Vdd, and
cathode to MCLR/Vpp) I can't see how the capacitor can hold MCLR low
during power up (which I assume is it's purpose when the ICSP is not
connected) ?

Peter

2005\06\17@180044 by Mike Hord

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> With that diode in place (anode to capacitor-and-resistor to Vdd, and
> cathode to MCLR/Vpp) I can't see how the capacitor can hold MCLR low
> during power up (which I assume is it's purpose when the ICSP is not
> connected) ?

How do you figure?  Diode or no, the rise time at that diode/R/C node will
be the same.  The only thing is, with the diode there, it has to go up a
little more before the PIC notices it (due to the drop over the diode)(which,
with a Schottky at that current, will be pretty low), so the time the PIC
is held in reset will actually be a little longer.

I'm not certain that's the reason for that capacitor, anyway.  And I'm
pretty sure I've heard more than once that most people don't put a cap
on MCLR/Vpp.

Mike H.

2005\06\17@182428 by Jinx

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> I'm not certain that's the reason for that capacitor, anyway.  And I'm
> pretty sure I've heard more than once that most people don't put a cap
> on MCLR/Vpp.

The PIC's power-up options mean a cap isn't usually necessary. I install
one in some applications to filter out Vcc noise that could cause a reset.
Any circuit with a reset switch has the full complement of recommended
components

2005\06\20@040114 by Michael Rigby-Jones

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{Quote hidden}

I can see what the OP is saying, with the diode in place, MCLR is essentialy free to float when the capacitor voltage is low and the diode is reverse biased.  There should really be a pull down on MCLR.

Regards

Mike

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2005\06\20@050836 by Peter Onion

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On Mon, 2005-06-20 at 09:00 +0100, Michael Rigby-Jones wrote:

>
> I can see what the OP is saying, with the diode in place, MCLR
> is essentialy free to float when the capacitor voltage is low
> and the diode is reverse biased.  
> There should really be a pull down on MCLR.

Yes !  That's exactly what I meant !  

Peter


2005\06\20@100914 by Mike Hord

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> > I can see what the OP is saying, with the diode in place, MCLR
> > is essentialy free to float when the capacitor voltage is low
> > and the diode is reverse biased.
> > There should really be a pull down on MCLR.
>
> Yes !  That's exactly what I meant !

Ah!  I see (said the blind man ;-)).

AFAICT, the problems associated with the Schottky/cap pair
in that circuit make it undesirable.  The other ugly fact is that
if one were to ground that MCLR pin, one would be left with a
capacitor charged to 5V grounded through that Schottky-
maybe not such a good thing.

I'm going to stick with the pullup only.

Mike H.

2005\06\20@102759 by Michael Rigby-Jones

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>> > I can see what the OP is saying, with the diode in place, MCLR is
>> > essentialy free to float when the capacitor voltage is low and the
>> > diode is reverse biased. There should really be a pull
>down on MCLR.
>>
>> Yes !  That's exactly what I meant !


>{Original Message removed}

2005\06\20@143416 by Rick Thompson

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{Quote hidden}

I don't see how MCLR woud be free to float when it's being driven by the
programmer.  But Mike brought up a good point in that the instantaneous cap
discharge current might be too great for some diodes, or at least it's not
good practice to design in the possibility.  A possible solution would be to
put a current-limiting resistor between the cap and the diode.  I personally
would want the cap in there for noise-induced resets.  Is there any reason
the cap can't be placed between the diode and the MCLR pin, provided it
doesn't overload the programmer?

Rick


2005\06\20@144941 by Hopkins

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The cap may slow down the MCLR signal causing incorrect MCLR operation
as far as the programmer is concerned.

_______________________________________

Roy
Tauranga
New Zealand
_______________________________________

-----Original Message-----

Is there any reason
the cap can't be placed between the diode and the MCLR pin, provided it
doesn't overload the programmer?

Rick



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2005\06\20@152713 by Peter Onion

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On Mon, 2005-06-20 at 11:34 -0700, Rick Thompson wrote:

> I don't see how MCLR woud be free to float when it's being driven by the
> programmer.

>From my original post that started the thread....

"With that diode in place (anode to capacitor-and-resistor to Vdd, and
cathode to MCLR/Vpp) I can't see how the capacitor can hold MCLR low
during power up (which I assume is it's purpose when the ICSP is not
connected) ?"


Note was asking about the situation when the programmer is NOT
connected.

Peter

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